The invention relates to a method for conveying a fluid and an apparatus, in particular a centrifugal pump, for conveying a fluid in accordance with the method of the invention.
A centrifugal pump is a conveyor machine in which mechanical energy is transmitted to a liquid with a rotating rotor wheel. The energy of motion is converted into pressure energy partly in the rotor wheel and partly in the subsequent guide apparatus. Depending on the angle of outlet of the fluid, a distinction is drawn between radial, half-axial and axial rotor wheels. Centrifugal pumps are built as single stage pumps or multistage pumps (with stacked rotor wheels arranged in series) and also as single flow or multiflow pumps (with parallel arrangements of the rotor wheels). The throughflow or delivery can be regulated by restricting the flow in the pressure line, by changing the rotational speed or, in the case of axial and half-axial rotor wheels, via adjustment of the :rotor blades. In single stage centrifugal pumps for larger conveying flows, pre-twist regulation is used by generating a twist with an upstream adjustable guide wheel.
The operational behavior of centrifugal pumps can be described by characteristic lines or curves as a function of the delivery flow Q. For example, the pressure head H can be described in terms of a H(Q)-curve. When the H(Q)-curve falls steadily as a function of the delivery flow Q, it is termed a stable characteristic. If the H(Q)-line has a positive gradient in the curve, this is termed an unstable characteristic. When operating the pump with an unstable characteristic, it is for example possible that a full-load region is not reached because, under some circumstances, the pump cannot be operated in the unstable region when being run up to speed. Moreover, it may be impossible to operate pumps with unstable characteristics in parallel. Furthermore,.flow dependent pulses can occur in an unstable region of the characteristic. The causes of an unstable characteristic are complex flow phenomena in the centrifugal pump.